UK6: Iodine concentrations in organic milk

Summary of final report:
Organic milk normally contains less iodine than conventional milk but the concentrations of iodine in organic milk are well within the optimal levels for human nutrition as it is evidenced by a number of European studies. Nevertheless, the relatively lower iodine concentrations in organic milk trigger discussions amongst stakeholders, farmers and researchers in view of the recent evidence that iodine intake has decreased due to a decrease in milk consumption in the UK.

The aim of the current project was to investigate the relationship between iodine concentrations in bulk milk samples with iodine concentrations in forage on organic dairy farms in view of farm practices. Ten organic dairy farms located in the west-south of England were selected and farmers agreed to participate in this study. The farms were categorised in terms of milk iodine concentrations as “low” (i.e below 60 μg/L), “optimal” (i.e between 60 to 120 μg/L) or “high” (i.e above 120 μg/L) and farmers agreed to a monitoring protocol that allowed data collection on iodine and other mineral concentration in milk, blood, urine and forage samples.

The results show that the monthly milk iodine concentrations averaged over the farms remained within optimal levels, but, in some farms milk iodine concentrations were systematically low through the monitoring period. Urine iodine concentrations were significantly higher in the farms with high (i.e. 1.5 mg/kg) or optimal (i.e. 0.5 to 0.8 mg/kg) forage iodine concentrations compared to the farms with low forage iodine (i.e < 0.5 mg/kg). This outcome reflects the well-established evidence that urine iodine is indicative of dietary iodine intake. With regards to milk iodine, this was not the case: farms with low or average forage iodine concentrations had higher milk iodine compared to the farms with high forage iodine concentrations.

Although this outcome is surprising, it reflects the fact that milk iodine concentrations are affected by the use of iodine-based teat disinfectants. Indeed, six out of the 10 case-study farms use iodised post-dip teat disinfectants, while the remaining 4 farms do not. Comparison between the two groups of farms indicated that milk iodine concentrations were 2.3 times higher in the farms that use iodised post-dip teat disinfectants (mean average 195 ± 13 μg/L) compared with the farms that do not use iodised post-dip teat disinfectants (mean average 85 ± 8.9). This outcome indicates that iodised post-dip teat disinfectants have a major positive effect on milk iodine concentrations and can wipe-out any effect that dietary iodine intake might have on milk iodine concentrations.
In conclusion, this study show that the use of iodised post-dip teat disinfectant is the most important influencing factor for the iodine concentration in milk and that where post-dip teat disinfectant is used the iodine concentrations in milk do not serve as a robust indicator in identifying shortfalls in iodine intake.

However, forage iodine concentration is an important factor in maintaining milk iodine concentrations at optimal levels, in addition to its importance in maintaining animal health and performance at optimum levels. Milk iodine concentrations fluctuated within farms across samplings but in some farms they were systematically low. This outcome deserves further attention in order to alleviate recent concerns that organic milk contains less iodine than conventional milk and to avoid that the health status of the animals might be negatively affected by low iodine intake. Where doubts about the iodine supply of animal exist, urine samples can be used to monitor the cow’s iodine status.